Methods for operating an internal combustion engine are known in which the lift of an intake valve of an engine cylinder is diagnosed. This is accomplished using optical or electronic sensor systems in the corresponding intake valve itself and thus involves considerable hardware complexity. Diagnosing the valve lift of the intake valves via the uneven running of the engine is also known. However, this only works in a satisfactory manner during idling and at higher speeds. Finally, diagnosing the valve lift of the intake valve via partial vibrations in the camshaft is also known. However, in this case it is impossible to determine which cylinder has the defective intake valve.